Periodic structures of nanometer dimensions but with long-range order (at least mms) inside transparent materials such as fused silica and other optical glasses have a variety of applications. Optical methods, such as femtosecond (1 fs=10−15 s) laser dielectric modification (FLDM), for fabricating such structures have advantages over traditional micro-fabrication techniques which for the most part only allow the fabrication on a surface and not inside the bulk material. In fact FLDM is the only viable technique capable of forming controllable 3-D structures inside bulk glasses (see “Methods for creating optical structures in dielectrics using controlled energy deposition” by Corkum et al, PCT WO/02/16070 A2 28 Feb. 2002, allowed as a US patent as of February 2005), and “Optically wiring discrete optical components and optical arrays using 3-D waveguide circuits written in transparent materials” by Taylor et al, U.S. patent application Ser. No. 10/826,312 filed 19th of Apr. 2004). FLDM can also be used directly to produce structures that have a refractive index contrast with the surrounding material or in combination with chemical etching to produce channels and shaped voids.
Traditionally it has been assumed that optical fabrication techniques are restricted by the diffraction limit of the light used. Typically, this means that optical techniques cannot be used to produce structures with dimensions of less than the order of a wavelength of the light (λ). Near-field optical approaches provide ways to avoid the diffraction limit but are restricted to forming nanostructures at surfaces and do not permit the fabrication of such structures within the body of a substrate. At high peak laser powers and near the threshold for modification of a dielectric modified zones can be formed that are somewhat smaller than the wavelength of light. The applicants have recently shown that an intrinsic property of FLDM under defined irradiation conditions is that the process can result in sub-diffraction limit structures. Femtosecond laser fabrication of nanostructures in silica glass”, Taylor, R. S.; Hnatovsky, C.; Simova, E.; Rayner, D. M.; Bhardwaj, V. R.; Corkum, P. B., Opt. Lett. 28, 1043-1045, (2003). There has also been some evidence in the literature that self-organized sub-features in the modified zone can be produced with a dimension of ≈20 nm that is dependent on the light polarization. Self-Organized Nanogratings in Glass Irradiated by Ultrashort Light Pulses, Y. Shimotsuma, P. G. Kazansky, J. Qiu and K. Hirao Phys. Rev. Lett., 91(24), 247405 (2003). The polarization dependent periodic nano-features were observed in fused silica within the material exposed to the static focus of a femtosecond laser i.e. neither the sample nor the focus were scanned. The prior art however does not teach how to harness these sub-structures to produce useful assemblies, especially long-range arrays.
Moreover, Shimotsuma et al. describe a model for the formation of the nanofeatures that is based upon the interference of the incident focused light with a bulk electron plasma wave formed by multiphoton ionization inside the dielectric material. The applicants believe this model is incorrect and leads to erroneous predictions. Shimotsuma et al. assume that a plasma is formed at very high electron densities very close to the critical density Ncr (i.e. when the plasma frequency=laser frequency) and at high electron temperatures (>1×107 K.). Interference between the incident light and the electron plasma wave gives rise to periodic bulk electron density variations in the plane of the laser polarization which leads to the observed nanostructures.
However high electron densities close to the critical density are not possible due to non-linear absorption and electron recombination which limits the density to ≈0.1Ncr. It is impossible to produce such densities because absorption would severely limit the length of the modified zones (<1 micron) whereas we see modified structures with lengths of tens of microns.
The required electron temperatures are unrealistic under his experimental conditions i.e. inside a dielectric where electron-ion collisions reduce the mean temperature <1×107 K.
A realistic 3-D treatment of Shimotsuma et al's 1-D model would result in electrons moving out of the focal volume in all directions, not with a unique velocity but with a distribution of velocities. This would make a host of plasma waves possible with different momentum vectors and there is no a-priori reason why the system would choose a particular plasma momentum vector to form a grating of specific spacing.
Shimotsuma et al obtain agreement between their model and their experimental data which shows a strong dependence of the grating spacing on the laser pulse energy. After analyzing considerably more data and much higher quality data than Shimotsuma et al, the applicants have observed that the spacing is independent of the pulse energy contrary to Shimotsuma et al and believe that their experiment is not representative.
The applicants have experimental data which shows that the spacing decreases linearly with a decrease in the laser wavelength. Shimotsuma has no experimental data on wavelength scaling but his model predicts a highly non-linear scaling relationship.
It is not intuitive how the spacing could be preserved from the Shimotsuma model when the focal spot is moved since there is no feedback mechanism apparent in the model. Indeed since it is a plasma mechanism it is not clear how the phase can be locked during sample motion.